Investigating Scour at Offshore Monopile Wind Turbine Foundations
When building an offshore wind farm, the effects of placing a vertical monopile foundation on the seabed can lead to a local change in water flow due to currents and waves. This can lead to scour, which can threaten the structural stability of the pile. Therefore, scour needs to be accounted for and considered in the early planning phases. The phenomenon of scour in sand is quite well understood and has led to industry bodies, namely Det Norske Veritas (DNV), publishing guidelines that predict a maximum scour depth of 1.3 times the pile diameter. Although followed, this standard does not take into account the sediment type and conditions in which foundations are being installed offshore. If industry guidelines were to be developed to account for wind farm conditions, costs (as well as carbon and energy) can potentially be saved.
Research Question: Is the design for an estimated scour depth of 1.3 pile diameters around offshore submerged pile foundations in over-consolidated clay overly conservative?
Many have acknowledged and commented on the complexity of modelling and predicting scour in clay but there has been little attempt to understand the problem. The effects of cohesion have been to increase the resistance to scour. Briaud et al (1999) also state that clays scour much more slowly than sand. Although laboratory testing has taken place on very soft clays with shear strengths of 2-12.5kPa by Rambabu et al (2003), Debnath et al (2010) and Briaud et al (1999), no data was found for over-consolidated clay. London Clay is over-consolidated due to historical glaciers, and has high shear strengths of 150-300kPa. The clay used in the experimental part of this research is in the range of 50-114 kPa.
This paper draws conclusions on scour in over-consolidated clay in two ways. Firstly, field data from offshore wind farms is analysed and wind farms in sand are compared to those in clay. In this analysis, there was evidence that the wind farms where clay was present, namely Barrow, Kentish Flats and North Hoyle, experience much less scour than those in sand, specifically Scroby Sands and Arklow Bank. Secondly, laboratory experiments were performed on a model monopile foundation. The experiment methodology the conditions in which a depth of 1.3 times the pile diameter was seen in sand and to recreate those conditions in clay. No scour was seen in clay with a shear strength above 50 kPa in the reference conditions until parameters were made more severe. Eventually, a scour depth of 0.3 pile diameters was seen in clay with a shear strength of 50 kPa but the conditions relative to those for sand were a 1.5 times higher flow velocity and half the water depth.